The present invention relates to miniature hand-held spot sources of illumination, and more particularly, the present invention relates to such types of illumination as are useful by health professionals to observe and operate on patients.
The modern dental operatory may be provided with as many as four different sources of illumination. These may include a window transmitting light from outdoors, an overhead generalized source of illumination, a directable source of illumination, and a hand-held spot source of illumination which may, and generally is combined with a dental handpiece mounting a drill or burr. The directable source of illumination is provided for the purpose of generally illuminating the oral cavity, as may be desired. An example of a directable light source is disclosed in U.S. Pat. No. 4,608,622 issued to Gonser on Aug. 26, 1986.
Miniature hand-held spot sources of illumination are generally incorporated in a handle of the hand-held dental instrument. One or more light beams are projected laterally of the handle from the proximal end portion thereof onto the drill or burr. Light from a lamp in the handle is transmitted to the light beam projectors via an optical light path of conventional construction. The air supply for the drill or burr operating turbine is routed through the handle alongside the optical path.
In approximately 90% of the dental operatories in the U. S., the directable source of illumination is provided by a tungsten halogen lamp which produces a light having a yellow-hue. This quality of light is not as desirable as either sunlight or light which has been color corrected such as in the manner disclosed in the aforementioned patent. A major reason for this is that yellow-red light blurs the distinctions between various internal layers of a tooth, such as between the enamel and dentin. In addition, in the diagnosis of gum diseases, variations in color of tissue can be important, and such variations are less distinct under a source of illumination having a yellowish hue.
To compensate for what is perceived as a lower level of illumination, some health professionals often resort to increasing the intensity of the illumination to enable them to observe the anatomical distinctions noted above. This approach, however, is not in the long term best interest of the health professional because it may lead to eye damage. The reasons for this may best be explained in the context of the physiology of the eye.
Structurally, the eye comprises a cornea, a lens and a chamber which is filled with a gelatinous, vitreous humor behind which lies the retina. The retina is composed of nerve fibers known as rods and cones. The cones are responsive to colors, but they are relatively insensitive to low intensities of light so that color cannot be distinguished readily at such low intensities, i.e. twilight vision. The rods, however, are more sensitive to light because they contain a light-sensitive substance, but they cannot transmit color sensation.
A network of fine fibers connected to the retina transmit sensory impression through the optic nerve to the brain. Perception of color by the brain is not instantaneous. For example, blue sensations of light are perceived more quickly, and green sensations less quickly.
In the eye, the highest visual acuity occurs in one small central area in the retina known as the fovea centralis. The fovea is the center of a larger, specialized region known as the macula lutea (yellow spot). There is a depression in the retina known as the fovea depression in which many of the retinal layers are reduced in thickness. It is termed the foveal pit. The foveal pit contains the highest concentration of cone receptors which are responsible, not only for color vision, but also for vision of increased acuity.
The cones are packed tightly in the fovea. For instance, in the 100 micrometer diameter of central fovea, there are approximately 7,000 cones. In contrast, there are approximately 100,000 cones in the entire rod-free region, and there are over 4 million cones in the macula. In the peripheral portions of the retina outside the macula, the receptors include mostly rods. In the entire retina, there are approximately 7 million cones and 125 million rods which feed into approximately one million nerve fibers contained in the optic nerve.
There are psychophysical and psychological aspects involved in color perception. For instance, there are three types of cones that are sensitive to the three additive primary colors: red, blue and green which compose so-called white light. Light energy is converted into chemical energy through the mechanism of the visual pigments. When the eye is focused on a colored object, the visual pigments are depleted, and if they are depleted faster than they can be regenerated, color discrimination is lost. It is important, therefore, for good long-term eye health that colors be viewed under a perfectly balanced light source which activates all of the specialized cones and minimizes the color pigment depletion problem. This enables visual acuity and color discrimination to be maintained over a longer period of time.
It is known that sunlight contains a full spectrum of visible light of various wavelengths, including the primary additive colors red, blue and green. So-called white light artificial illumination, to be comparable to sunlight, should also contain a proper balance of light waves.
Systems have been devised for comparing the quality of light provided by artificial sources of illumination. Such a system includes reference to the term correlated color temperature (CCT). Correlated color temperature is the color temperature of a source of illumination which is equal to the temperature of a black body radiator heated to the temperature (in degrees Kelvin) necessary to produce that color. During heating, the black body undergoes a series of progressive color changes. For instance, initially, it glows red, then glows orange, yellow, white and finally blue respectively. Since an artificial light source does not actually generate the same thermal temperature as those corresponding to a black body possessing a particular color, the term correlated color temperature has been developed. For example, a candle glows at a correlated color temperature of 2,000 degrees Kelvin; a tungsten-halogen lamp glows at a correlated color temperature of about 3,200 degrees Kelvin; and a slightly overcast sky has a correlated color temperature in a range of about 5,500 to about 6,000 degrees Kelvin.
A properly balanced white light is characterized by the absence of hues. A system has been developed for quantifying the quality of light. This system includes a so-called color rendering index (CRI) and may be found in an algebraically derived chromaticity diagram. There is a point on the diagram representing a perfect balance of the various components of light, and this is referred to as an equal energy point. It is given a color rendering index of 100. Light with various hues, i.e. a red hue, a blue hue, etc. are charted at other locations on the chromaticity diagram. Hued light has a color rendering index of less than 100, depending on its distance from that point on the diagram. It has been determined that the highest efficiency of eye response in the dental operatory for the examination of anatomical structures of teeth and gingivas occurs when the color rendering index of the light used for examination purposes is 90 or greater.
While the quality of light required for anatomical examination purposes as described above is important, the quantity of light is also important. The dental operatory, for example, should have ambient lighting of at least about 150 footcandles (fc.). The source of illumination for the oral cavity, i.e. the directable source of illumination, should not have an intensity which exceeds about ten times that of the ambient light level, or about 1,500 footcandles. Preferably, the ratio between the ambient lighting in the dental operatory and the directable source of illumination is about three to one, and the corresponding ratio between the operatory ambient light and spot illumination in the oral cavity is about two to one, i.e. about 150 footcandles ambient lighting, 450 footcandles directable operatory lighting, and 900 footcandles for intracavity spot illumination. The levels of illumination may, of course, vary from these depending upon the particular preferences of the health professional. For the best long-term eye health of the professional, of course, it is highly desirable to minimize light intensity without compromising observability.
In addition to quality and quantity of light, contrast is important in the oral cavity. For example, the typical directable dental operatory light provides illumination in the oral cavity in a color correlated temperature range of between about 2,900 and 3,800 degrees Kelvin which is the color temperature of the tungsten halogen lamp source. Approximately 90% of all such sources of illumination installed in the U. S. provide a yellowish illumination characteristic of such a source. Spot illumination from the dental handpiece of the same correlated color temperature tends to blend with the overall oral cavity illumination, and this is undesirable. When, however, the correlated color temperature of the spot source is about 5,500 degrees Kelvin, good contrast is provided.